1. Field of the Invention
This invention relates to mechanisms that prevent the automatic turn-on of hazardous machinery upon inadvertent interruption and subsequent supply of electrical power to the machinery.
2. State of the Art
Hazardous machinery such as hand-operated power tools (e.g., routers, planers, saws, etc) and food slicers and mixers typically have an integral ON-OFF toggle switch that is manually-controlled by the operator. A safety issue arises when power to the machinery is unintentionally interrupted while the machinery is being operated (for example, caused by the power plug of the machine being accidentally pulled from its outlet or a failure of the mains power) and then the machine restarts unintentionally when power is restored and the ON-OFF toggle switch remains on.
U.S. Pat. No. 4,258,368 proposes a mechanism to deal with this problem. This mechanism includes a triac switch S2 and run switch S3 that cooperate to ensure that the machine's motor does not restart unintentionally when power is interrupted and subsequently restored and the ON-OFF toggle switch S1 remains on. More specifically, the triac switch S2 is activated during normal operation of the motor by a control signal generated by a pickup coil magnetically coupled to a rotor assembly of the motor. The run switch S3, which is normally-open and manually-actuated, provides a shunt path around the triac switch S2 when the run switch S3 is momentarily closed. The mechanism works as follows. When the machine is plugged into a working wall receptacle, the machine's motor is energized by closing the toggle switch S1 and then closing the run switch S3. When switch S3 is closed, the triac switch S2 is shunted and current flows through the lead to a pickup coil, which energizes the main winding and starter winding of the motor. As soon as current begins to flow though the main winding, a voltage signal is induced in the pickup coil which closes the triac switch S2. With the triac switch S2 closed, the motor operates normally regardless of the release of the run switch S3. When power is inadvertently interrupted and subsequently supplied to the machine, the triac switch S2 prevents automatic startup of the motor. To restart the motor, the operator must ensure that the toggle switch S1 is ON (closed) and then must manually actuate the run switch S3 so as to shunt the triac switch S2 to thereby enable the motor to start. This mechanism has many disadvantages. First, it requires that an additional coil (the pickup coil) be added to the motor, which is expensive to implement in new products and is difficult to integrate as an add-on to existing products. Moreover, it is cumbersome to use, requiring the operator to manipulate both the ON/OFF switch S1 and the run switch S3 to start and restart the motor.
Thus, there remains a need in the art for an improved electrical lockout mechanism that prevents such unintentional machine restarts yet minimizes the tasks performed by the operator under normal operating conditions. Moreover, there remains a need in the art for an improved electrical lockout mechanism that is easily and inexpensively adapted for use in both new products and existing devices.